The ancient evolutionary origin of the elusive female orgasm

By Susan Scutti, CNN

Updated 12:28 PM ET, Tue August 2, 2016

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Photos:Hollywood's steamiest sex scenes

'Fifty Shades of Grey' (2015) – "Fifty Shades of Grey" came out around Valentine's Day in 2015, a perfect time for a film whose sex scenes had some blushing red. It isn't the first movie with risque scenes that had moviegoers titillated.

'Don't Look Now' (1973) – The graphic sex scenes between Donald Sutherland and Julie Christie in "Don't Look Now" resulted in an X rating at first and have fans still wondering, did they or didn't they?

'Love Jones' (1997) – "Love Jones" is the story of a poet and photographer -- Nia Long and Larenz Tate -- trying to figure out if they'd found lasting love, and it's filled with scenes of their lustful explorations. Their initial sleepover, set to the tune of Maxwell's "Sumthin' Sumthin'," is one that'll threaten to set fire to your TV.

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Photos:Hollywood's steamiest sex scenes

'Wild Things' (1998) – Matt Dillon and Denise Richards helped steam things up in the erotic thriller "Wild Things," which included a menage a trois scene with Neve Campbell.

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Photos:Hollywood's steamiest sex scenes

'Eyes Wide Shut' (1999) – Tom Cruise and Nicole Kidman were still married when they co-starred in "Eyes Wide Shut," in which iconic director Stanley Kubrick pushed the envelope. Years later, there is still talk about hidden messages.

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Photos:Hollywood's steamiest sex scenes

'Cruel Intentions' (1999) – The cast of "Cruel Intentions" had chemistry all the way around, but the heat between the once-married Reese Witherspoon and Ryan Phillippe was palpable.

'The Notebook' (2004) – "The Notebook" has become the gold standard for romantic movies, but one scene in particular will have viewers pausing to rewind. When Ryan Gosling's Noah and Rachel McAdams' Allie Calhoun reunite, not even pouring rain can dampen the sensual scene that follows.

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'Brokeback Mountain' (2005) – Heath Ledger, left, and Jake Gyllenhaal share a cowboy love that dare not speak its name in "Brokeback Mountain."

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'A History of Violence' (2005) – Viggo Mortensen and Maria Bello are a loving couple who face danger in "A History of Violence." But they also don't mind spicing it up with costumes and role play.

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'Black Snake Moan' (2006) – If there's any reason to see "Black Snake Moan," it's to see just how well Christina Ricci and Justin Timberlake work together in their love scenes.

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'Shortbus' (2006) – Raphael Barker and Sook-Yin Lee star in "Shortbus," about a female sex therapist and a group of sexually adventurous New Yorkers.

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'The Reader' (2008) – Kate Winslet and David Kross played out the heat between a younger man and an older woman in "The Reader."

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'Blue Valentine' (2010) – Ryan Gosling and Michelle Williams did such excellent work portraying sometimes-volatile lovers in "Blue Valentine" that the Motion Picture Association of America initially tried to give the movie an NC-17 rating.

'Blue is the Warmest Color' (2013) – Adele Exarchopoulos, left, and Lea Seydoux star in "Blue is the Warmest Color," which drew lots of attention upon its release because of its graphic sex scenes, including one that is more than six minutes long.

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'The Wolf of Wall Street' (2013) – Margot Robbie and Leonardo DiCaprio could make pretty much anything look amazing, including enjoying one another on top of a pile of money. Turns out that steamy scene wasn't as fun for the actress: "If anyone is ever planning on having sex on top of a pile of cash: don't," Robbie said in an interview with The Daily Beast.

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The hormonal surge women experience during orgasm once stimulated ovulation, research suggests

(CNN)What is the purpose of the ever-elusive female orgasm?

In males, orgasm is invariably required for ejaculation and transfer of sperm, researchers noted in a piece published recently in the Journal of Experimental Zoology. "But in females its function is unclear," wrote Mihaela Pavlicev, a researcher at Cincinnati Children's Hospital Medical Center,and Gunter Wagner, a professor of ecology and evolutionary biology at Yale University.

In women, the orgasm does not contribute to reproductive success and accompanies intercourse only "unreliably," they said. So, what gives?

The researchers examined many species of mammals and proposed that orgasm in women evolved from an ancient trait that once played a key role in reproduction.

To discover the big O's reason for being, the researchers explored its underlying physiology and focused on the hormonal surge that women experience while in the throes of passion. Keeping this foremost in mind, Pavlicev and Wagner then looked at other mammals. Despite the enormous diversity of reproductive styles, they were able to identify core characteristics of reproduction and track how these features evolved.

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The evolution of ovulation

Certain species of mammals are reflex ovulators. For example, female cats produce a mature egg ready to be fertilized by sperm only when stimulated by intercourse with a male. By comparison, women are spontaneous ovulators who produce eggs monthly, independent of sexual intercourse.

Looking at the distribution of these two types of ovulation across all the species of mammals, Pavlicev and Wagner inferred that male-induced ovulation must have evolved first and spontaneous ovulation later.

If female orgasm, by causing a hormonal surge, helped induce ovulation, this means it once played a direct role in reproduction. Over time, though, spontaneous ovulation evolved, rendering the female orgasm unnecessary.

Looking at the evolution of female genitals, the researchers found additional evidence that orgasm once played a fundamental role in reproduction. As females moved away from reflex ovulation toward spontaneous ovulation, the clitoris -- the major source of orgasm -- moved away from the location of sexual intercourse, the vagina.

All this adds up to broader implications for women, Pavlicev and Wagner suggested: The low frequency of female orgasm -- only a quarter of all women reliably achieve orgasm during intercourse -- is not an individual woman's failing or a physical impossibility but a natural consequence of evolution.

Others challenged this theory.

Heated views

Elisabeth Lloyd, faculty scholar at the Kinsey Institute for Research in Sex, Gender and Reproduction, found this new theory to be "an impressive piece of work," yet she also had a "quibble." (To help prepare the paper for publication, Lloyd provided comments for the authors but was not involved in the research.)

The researchers define orgasm completely in terms of hormonal surges, she said, but "there's a lot more to female orgasm than hormonal surges." The standard definition of female orgasm used by scientists includes pelvic muscle contractions, she added.

In her own book published in 2005, "The Case of the Female Orgasm," she examined "all 21 published theories" of female orgasm at that time -- yes, 21 -- and discovered they all basically fell into a couple of categories.

The "byproduct theory" -- the one Lloyd prefers -- is based on the biological fact that males and females develop similar traits in the first two months of life as our basic body plan and tissue patterns get laid down.

"Males get nipples because females need them, and females get orgasms because males need them," she said.

By contrast, the pair-bond theory, which is adaptation-based, understands female orgasm to have evolved for the purpose of strengthening the relationship between a male and a female. In other words, the pleasurable feeling of an orgasm would encourage a woman to return to the man for more.

"Adaptation is a trait that evolves through being selected for its advantages and its contribution to evolutionary fitness," Lloyd explained, adding that any adapted trait would therefore contribute to the number of offspring a person leaves behind.

If female orgasm is an evolutionary adaptation, then you would expect to see more orgasmic women reproducing more children and passing that trait on to their children. But that isn't the case, Lloyd explained, citing one study of more than 8,000 women in which the researchers found no correlation between female orgasm and the number of her offspring. Women who often reach orgasm don't necessarily have more kids than other women.

Could the female orgasm still have a purpose?

"Lack of evidence of an adaptive role for women's orgasm should not be taken as evidence that it is simply a byproduct or 'side effect' of men's orgasm and that it has no role," said Barry Komisaruk, a professor in the psychology department at Rutgers University. Because a woman's orgasm has properties that are unique -- different from a man's orgasm -- it probably has a unique, if unknown, function, he added.

While Komisaruk has spent years investigating the female orgasm, his interest developed from an unlikely start.

The film "When Harry Met Sally" featured a famously fake female orgasm.

"I was studying a reflex called the pseudopregnancy reflex in rats," he explained. As first demonstrated in the 1920s, female rats who mate with a vasectomized male rat will develop a false pregnancy that still includes a change in hormones and even the development of a placenta.

After studying brain cell activity of anesthetized rats, Komisaruk developed a method of studying neurons in awake rats. Here, he discovered the same vaginal stimulation that led to a pseudopregnancy also appeared to decrease pain in female rats.

While this seemed to be the case based on their reaction to a pinched paw, how do you really know whether a rat is feeling less pain during vaginal stimulation? With the help of his former graduate student Beverly Whipple, Komisaruk answered this question by performing experiments with women as subjects and simply asking them.

Komisaruk and Whipple discovered the power of the female orgasm. A woman's pain threshold might increase by 50% -- for some women, by more than 100% -- during orgasm.

Naturally, Komisaruk wanted to know more. Mainly because no one else had ever done so, he mapped what happened in the brain during these moments of ecstasy.

"Virtually every brain system is activated during women's orgasms," he said. "It's such a big phenomenon, it must be doing something."

According to Komisaruk, Pavlicev and Wagner "make an unfounded link between reflex ovulation and orgasm" while making "an unfounded claim that ovulation is induced by clitoral stimulation."

The female orgasm may have importance in mate selection, not only for women but also for men, Komisaruk said. Feeling sexual pleasure, a woman decides to stand by her man; meanwhile, "the feeling of efficacy" a man gets when he elicits the female orgasm becomes "a potential strong reinforcement for the man," he said, much in line with those who argue in favor of pair-bonding.

For Komisaruk, a "safer working hypothesis" is that if a process exists, it's probably not a mistake or a vestige, there's a function for it.

Understanding a 'powerful phenomenon'

Originally, Lloyd confessed, she believed much the same as Komisaruk: that the female orgasm had to be an adaptation. However, the numbers changed her mind.

"It made an impression on me that only 25% of women have orgasm reliably with intercourse and a third of women never or rarely have orgasm with intercourse," Lloyd said. This distribution across the population suggested anything but an adaptation.

Meanwhile, Komisaruk argues that female orgasm is pleasurable and reinforcing and, most important of all, activates the nucleus accumbens -- the same part of the brain that drives motivation and is activated by anything pleasurable, including opiates.

"How can such a powerful phenomenon not play a part in our behavior -- a crucial factor in evolution?" Komisaruk asked.